A rectifier is a converter which rectifies alternating current from alternating m-phase line current into direct current, with smoothing inductances on the D.C. side or generally with such impedance conditions, that this rectifier operates as a constant current source. An inverter is a converter which transforms direct current from a current source into alternating current, for the loads such as alternating current machines or network represented by a combination of RLC elements. The main difference between a rectifier and an inverter is the fundamental direction of flow of electric power. In a rectifier power flows from the alternating current side to the direct current side. In an inverter it is from the direct current side to the alternating current load. It is possible and in practice quite frequent, that at certain conditions of operation, the flow of power is reversed.
Where rectifiers and inverters are connected to a network, the fundamental method of control of the output is phase control and the fundamental process in the course of operation of an inverter is external commutation. The passage of current from one main branch of a converter to the following--the commutation--is controlled by external voltage. The commutation time, i.e. the time of transfer of current is determined by the angle of phase control, by the magnitude of current and by impedance conditions at the source and load. Semiconductor converters are mostly used as rectifiers with external commutation and the operation of external commutation and the other properties of these converters are commonly known and described in detail in technical papers.
Rectifiers and inverters with internal commutation have the same fundamental properties as similar converters with external commutation. The current commutation, however, does not proceed spontaneously by action of external voltage, but is forced by auxiliary circuits of the converter. The cause thereof is, for instance, a passive load on the alternating side of the inverter, or such a phase position of the alternating voltage at the moment of commutation, that the alternating voltage cannot cause an external commutation. This is rather linked to the direction of flow of idle power. It can be generally said that external commutation is possible where the alternating side is capable of furnishing idle power, for instance with a supply network. Internal commutation is possible in all other cases, for example with passive load, an electric motor or a supply network. Internal commutation can be introduced with all types of loads. It is above all a consequence of how complicated the converter is and how high the respective costs are, that at present rectifiers or inverters with external commutation prevail.
The most significant practical application of rectifiers or inverters with internal commutation are inverters for driving asynchronous motors. It can be supposed that a further important application will be a rectifier or an inverter on a network for power factor compensation, possibly also D.C. drives with improved energy parameters.
Known rectifiers or inverters, without regard to different alternatives of auxiliary commutation circuits, have one common property. It is the characteristic course of the commutation process, which has practically the same properties in entirely different commutation circuits. It is therefore necessary for simplicity to introduce the term "degree of commutation". The degree of commutation is the number of transitions between branches, between main and auxiliary branches, of the converter, which is required for execution of the complete commutation. A complete commutation in rectifiers and inverters involves the complete transfer of current from one main branch to the following main branch. In practice it means the transfer of current from one phase of the alternating side to the following one.
Direct commutation, where electric current passes directly from one main branch to the following, for instance common external commutation, is, from this point of view, a single stage commutation.
Indirect commutation, where electric current in a transfer from one main branch to the following one commutates at first to an auxiliary branch of the converter is at least a two stage commutation. For voltage inverters and also for other types of converters with internal commutation, for instance in pulse converters, the internal commutation proceeds mostly in two stages, and is therefore a two stage commutation.
Thus, the first stage of a two stage commutation is commutation from the main branch to the auxiliary one and generally includes a commutation capacitor, reactors and auxiliary thyristors. The second stage of the two stage commutation is a commutation from the auxiliary branch to the following branch, or main branch, for instance to a return diode of a voltage inverter and to a pulse converter or to a main thyristor of the following phase of a current inverter.
Current inverters with internal commutation are at present used predominantly for speed regulation of asynchronous motors. In known power circuits with thyristors the commutation process proceeds in two stages. The first stage commutation is from the main thyristor to the auxiliary circuit of the commutation capacitor without change in the load current of the phase. In the second stage, commutation is from the auxiliary circuit of the commutation capacitor to the following phase. The transfer of load current between phases proceeds only during the second stage.
To clarify the concept of two stage commutation, the operation of a known two stage commutation system will be discussed with reference to FIGS. 1 and 2. In FIG. 1, a three-phase load L.sub.r, L.sub.s, and L.sub.t is shown. A DC current, I.sub.d, is supplied to the system. A first main input terminal of the circuit is connected to the anodes of the main thyristors V1, V3, V5 and auxiliary thyristors V11, V13 and V15. The cathodes of thyristors V1, V3 and V5 are connected to one terminal of loads L.sub.r, L.sub.s, and L.sub.t, respectively, the other terminals of the loads being connected in common. The cathodes of thyristors V1, V3 and V5 are also connected respectively to the anodes of thyristors V4, V6 and V2, whose cathodes are connected in common to the second main terminal. Commutating capacitors C1, C2 and C3 are connected from the cathodes of thyristors V11, V13 and V15 to the cathodes of thyristors V1, V3 and V5, respectively. The cathodes of auxiliary thyristors V11, V13 and V15 are connected to the second main input terminal through thyristors V14, V16 and V12, respectively.
Let it first be supposed that current flows via V1 to phase R of the load, and returns via phase T of the load and V2. At time t0, auxiliary thyristor V11 and thyristor V3 are switched to the conductive state. Capacitor C1 is charged to the polarity indicated in FIG. 1. After thyristor V11 has been switched to the conductive state, the current passes from the path including V1 to the path including V11 and C1. This process has finished at time t.sub.1. Capacitor C1 is charged up to time t.sub.2 by the load current, cutoff voltage being applied to thyristors V1 from time t.sub.1 to time t.sub.2 (see FIG. 2).
From time t.sub.2 to time t.sub.3 the capacitor charges in the opposite direction, i.e. to a polarity opposite that shown in FIG. 1. At time t.sub.3, the voltage across capacitor C1 is equal to a value U.sub.sr, that is, it is equal to the instantaneous voltage between terminals R and S of the load. At this point, thyristor V3 begins to conduct and commutation takes place from the path including V11, and C1 of phase R to the path including V3, phase S of the load. Thus, the commutation of current in the load takes place within the time interval t.sub.3 to t.sub.4. During this time, the capacitor C1 is charged to a value such that the voltage across it exceeds voltage U.sub.sr by a voltage .DELTA.U given by the following equation: ##EQU1## which--under the condition of L.sub.R =L.sub.S =L--can be put as ##EQU2##
Thus the final voltage value on the capacitor is EQU u.sub.C =U.sub.SR +.DELTA.u
It will be noted that the above described two-stage commutation has the following disadvantages:
Since the commutation circuits are used both for disconnecting the thyristors and for accumulating power from the inductance of the load, the capacitors must fulfill two rather different functions with partly opposing requirements. Specifically, it is desirable that the time interval t.sub.1 to t.sub.2 in FIG. 2 correspond to the blocking time of thyristors, namely approximately 50 .mu.sec. This leads to a relatively small capacitance value. The low capacitance value would then lead to such high overvoltage on the capacitor and on the thyristors that voltage damage could occur. Thus, while reduction of the capacity of the capacitor optimizes the blocking of the thyristors, for purposes of accumulation of energy, a larger capacitance value would be preferable.
Secondly, the commutation capacitor must be matched to the load, that is changes in the commutation circuit and, in particular, in the commutation capacitor would be required for different motor loads. This, of course, is a great disadvantage.